It is known that a very large majority of pieces derived from steel bars and billets is obtained by press-forging, after shearing, and preferably by hot-pressing at temperatures of 1100.degree.-1300.degree. C. In such a way it is possible to reach satisfactory results, not only cost-wise, but also concerning the quality of the finished product, compared to cold processing both on a machine tool and through shearing and pressing at temperatures lower than 1100.degree. C.
It is also known that, for heating steel bars and billets stocked at ambient temperature, to the above mentioned temperatures, up to now use has been made of combustion furnaces or induction furnaces. In the former (also in chronological terms) liquid or gaseous hydrocarbons (presently methane is preferred) are used to generate heat. Heat transfer to the products forwarded therewithin, for instance by means of pilger rolls or beams, takes place by radiation from the ceiling and from the walls of said furnace wherein the products are introduced through the front or through the sides, and are retained for a predetermined time span, so that they reach, at the exit therefrom, the predetermined temperature. If the latter has to reach values of 1150.degree.-1300.degree. C. for hot-pressing, as it has been mentioned above, very thick and expensive refractory linings must be provided, which strongly increase the furnace thermal inertia whereby, when the furnace requires maintenance operations, rather frequent at said high temperature levels, very long waiting times are needed for cooling, in the order of several days, during which production has to be stopped. A further problem taking place when combustion furnaces are used to reach the hot-pressing temperature directly, besides the fact that the energy performance drops considerably above a certain temperature, is due to the strong oxidation and associated formation of surface scale taking place on the products in particular due to the long high temperature residence times which cause as well the problem of a possible material decarburizing. It should further be noted that these times are further increased when, for any reason like a shears failure, forwarding of the pieces is interrupted.
On the other hand, also the usage of the induction furnaces alone to reach hot-pressing temperatures starting from ambient temperatures causes some important drawbacks, like the fact that furnaces having such performance are necessarily expensive, although they provide some advantages like the fact that the furnace goes almost immediately to rated conditions with an optimum thermal control capacity, reduction of scale owing to the shortened high temperature residence time, as well as the reduced maintenance requirements with an associated shortening of the operation downtimes.
One of the reasons why use of the induction furnaces has not developed in proportion to what the advantages mentioned above would suggest, besides the costs recalled above, is the fact that steel has a Curie temperature level around 760.degree. C., above which a ferromagnetic material becomes a magnetic, whereby the relative magnetic permeability value (.mu.r) becomes equal to 1. Therefore, there takes place a substantial heating inductor performance reduction in that the power transferred to the piece to be heated is given by the following formula: EQU Pw=.mu.o.times..mu.r.times..pi..times.F.times.H.sup.2 .times.V.times.K
wherein:
Pw=power transferred to the piece located within the heating inductor; PA1 .mu.o=absolute permeability of air=4.pi..times.10.sup.-7 ; PA1 .mu.r=relative permeability=average value 20 below the Curie temperature;. PA1 F=working frequency in Hz; PA1 H=magnetic field intensity in Asp/m; PA1 V=volume of the piece to be heated; PA1 K=a function of the ratio between diameter of the piece and current penetration depth inside the piece. PA1 .rho.c=resistivity of the inductor material=0,017.times.10.sup.-6 .OMEGA..times.m; PA1 .rho.W=resistivity of the piece being heated. In the case of a 0,23% carbon steel we have a value at 20.degree. C. of 0,160.times.10.sup.-6 .OMEGA..times.m; a value at 1200.degree. C. of: 1,22.times.10.sup.-6 .OMEGA..times.m; PA1 .mu.r=relative permeability=average value 20 below the Curie temperature.
The inductor performance may also be defined through the following formula: ##EQU1## wherein: .eta.=inductor performance;
Therefore, it would seem to be preferable to use induction furnaces only for heating to a level below the Curie temperature. If it were necessary to reach higher temperatures a combustion furnace should be used. Therefore, it would seem to be advisable to use induction furnaces until the Curie temperature is reached, passing then to a combustion furnace.
On the contrary FR-A-2,284,847 discloses a steel bar and billet heating system for further processing, which provides an initial heating, from ambient temperature up to 700.degree. C. in a combustion furnace, followed by heating up to 1200.degree.-1250.degree. C. in induction furnaces. Also "Steel in the USSR", Vol 12 No. 3 March 1982, London (pages 131-133) shows a similar system wherein the combustion furnace is used for heating up to 750.degree.-800.degree. C. and an electric heating up to the temperature of plastic working.
From U.S. Pat. No. 4,559,854 a shearing equipment is known in which induction furnaces are located immediately before the shears and a roller path is provided between the induction furnaces for forwarding the bars and billets towards said shears.